Surface-to-borehole (STB) and borehole-to-surface (BTS) electromagnetic (EM) measurements may be sensitive to the presence of resistive targets. For example, various examples of STB and BTS numerical simulations exist where the response of the target provides useful resolution of hydrocarbon reservoirs in exploration and monitoring scenarios. However, a metal casing in the STB or BTS borehole may provide a highly conductive medium that yields current channeling along the length of the casing. This may result in an additive contribution to measured EM fields due to currents induced on, and leaking out of, the surface of the casing.
Finite size, three-dimensional, conductivity inhomogeneities in the shallow subsurface, known as karsts, may also yield channeling of the current density. The channeling phenomena may be described to first order as an accumulation of charges at the boundaries of the 3D karsts, which may effectively yield secondary sources at the position of the karsts. In turn, the secondary sources may bias the EM fields measured away from the primary source, both on the surface of the Earth as well as downhole.
Overburden structures may also effect STB and/or BTS EM measurements. For example, hydrocarbon targets may be located at depth, and little or no information about electrical properties of the overburden may be collected. However, the resistivity distribution of the overburden may affect the ability to extract accurate geological information at depth. Lack of knowledge about the overburden resistivity may introduce geological noise to STB and/or BTS data, which in turn may affect the ability of imaging hydrocarbon targets.